The present invention relates to expandable endoprosthesis devices, generally called stents, which are adapted to be implanted into a body lumen of a patient, such as a blood vessel, to maintain the patency thereof. These devices are useful in the treatment and repair of atherosclerotic stenoses in blood vessels.
Stents are generally cylindrically-shaped devices which function to hold open and sometimes to expand a segment of a blood vessel or other anatomical lumen. They are particularly suitable for use to support and hold back a dissected arterial lining which, if not so supported and held, can occlude the fluid passageway therethrough.
A variety of devices are known in the art for use as stents and have included: coiled wires in an array of patterns that are expanded after having been placed intraluminally via a balloon catheter; helically-wound coiled springs manufactured from an expandable heat sensitive metal; and self-expanding stents inserted in a compressed state and shaped in a zig-zag pattern. Some more examples are shown in U.S. Pat. No. 4,776,337 to Palmaz; U.S. Pat. No. 4,655,771 to Wallsten; U.S. Pat. No. 4,800,882 to Gianturco; U.S. Pat. No. 4,913,141 to Hillstead; and U.S. Pat. No. 5,292,331 to Boneau.
Such prior art devices include an expandable intraluminal vascular graft that is expanded within a blood vessel by balloon associated with, typically, a dilatation catheter. The graft may be a wire mesh tube, a stainless steel tube with rectangular openings, or a tube with honeycomb style openings. Another prior art device includes a prosthesis for transluminal implantation comprising a flexible tubular body made of flexible thread elements wound together, each thread having a helix configuration.
There are still more conventional endovascular stents. In one design, the wire stent has a generally cylindrical shape, wherein the shape is formed with alternating bent wire loops. Another conventional stent design comprises a series of continuous corrugations compressed together to form a tube-like mesh. Yet another endovascular stent used for the treatment of restenosis is a unitary wire structure, shaped to criss-cross and form a plurality of upper and lower peaks.
One of the difficulties encountered using prior art stents involved maintaining the radial rigidity needed to hold open a body lumen while at the same time maintaining the longitudinal flexibility of the stent to facilitate its delivery. Another problem area was the limiting range of expandability. Certain prior art stents expanded only to a limited degree due to the uneven stresses created upon the stents during radial expansion. This necessitated providing stents having a variety of diameters, thus increasing the cost of manufacture. Additionally, having a stent with a wider range of expandability allowed the physician to re-dilate the stent if the original vessel size was miscalculated.
Another problem with the prior art stents was that the stent contracted along its longitudinal axis upon radial expansion of the stent. This caused placement problems within the artery during expansion.
Various means have been devised to deliver and implant stents. One method frequently described for delivering a stent to a desired intraluminal location involved mounting the expandable stent on an expandable member, such as an inflatable balloon. The balloon was provided on the distal end of an intravascular catheter. The catheter was advanced to the desired location within the patient""s body lumen. Inflating the balloon on the catheter deformed the stent to a permanently expanded condition. The balloon was then deflated and the catheter removed.
What has been needed and heretofore unavailable is a stent which has a high degree of flexibility so that it can be advanced through tortuous passageways and can be radially expanded over a wide range of diameters with minimal longitudinal contraction, and yet have the mechanical strength to hold open the body lumen into which it is expanded. There is further a need for a stent-that has high circumferential or hoop strength to improve crush resistance. The present invention satisfies these needs.
The present invention is directed to an expandable stent having a configuration generally of the type disclosed in U.S. Pat. Nos. 5,569,295 to S. Lam and 5,514,154 to Lau et al., the entire contents of which are incorporated herein by reference. In a preferred embodiment, the present invention stent includes a plurality of adjacent cylindrical elements which are expandable in the radial direction and which are arranged in alignment along a longitudinal stent axis. The cylindrical elements are formed in a serpentine wave pattern transverse to the longitudinal axis and contain a plurality of alternating peaks and valleys.
The present invention also comprises at least one interconnecting member that extends between adjacent cylindrical elements and connects adjacent cylindrical elements to each other. The interconnecting members insure minimal longitudinal contraction of the stent during radial expansion of the cylindrical elements.
The present invention further comprises, in each cylindrical element, a reinforcing member that extends across each peak and valley. More precisely, each peak and each valley of a single cylindrical element is formed by the confluence of two straight struts joining at a bend. The reinforcing member thus spans across the peak or valley, bridging the struts.
The reinforcing member lends strength to the alternating peaks and valleys, wherein the area of maximum stress is at or near the bend. To be sure, the reinforcing member prevents the straight section of the strut from buckling or distorting during expansion of the stent by adding material to a potentially weak area. Furthermore, the size and geometry of the reinforcing member along with the bend may be adjusted so that stress is evenly distributed between the two instead of just being carried by the bend.
Certainly the geometry of the reinforcing member in the present invention can assume many configurations. For example, the reinforcing member could include a loop that curves toward or away from the bend. The reinforcing member could join the struts at a point farther away from or closer to the bend. The reinforcing member can be formed into the bend.
The resulting stent structure is preferably a series of radially-expandable cylindrical elements that are spaced longitudinally close enough to each other so that small dissections in the wall of a body lumen may be pressed back into position by the elements against the lumenal wall, but not so close as to compromise the longitudinal flexibility of the stent. The individual cylindrical elements may rotate slightly relative to adjacent cylindrical elements without significant deformation, cumulatively providing a stent which is flexible along its length and about its longitudinal axis, but which is still very stable in the radial direction in order to resist collapse.
The stent embodying features of the present invention can be readily delivered to the desired lumenal location by mounting it on an expandable member of a delivery catheter, for example a balloon, and by then passing the catheter-stent assembly through the body lumen to the implantation site. A variety of means for securing the stent to the expandable member on the catheter for delivery to the desired location are available. It is presently preferred to compress the stent onto the balloon. Other means to secure the stent to the balloon include providing ridges or collars on the inflatable member to restrain lateral movement, or using temporary, bioabsorbable adhesives.
The present invention by use of the reinforcing members features bimodal deployment. That is, when the stent is expanded radially as described above, it does so in two stages. The first stage is the type of expansion of the stent radially wherein the struts bend slightly outward to accommodate the increasing circumference of each cylindrical element and the loop portion of the reinforcing member is stretched out. The second stage continues from the first stage with the struts continuing to bend outward and with the most severe bending occurring at the reinforcing member until the struts are pulled wide apart to their limits to accommodate the largest diameter that the stent can assume. Further spreading apart of the struts is prevented by the presence of the reinforcing member, which limits the maximum circumferential size attainable by each cylindrical element. By choosing the size and geometry of the reinforcing member and the struts, the amount of force needed to expand the stent to a particular diameter can be altered.
The cylindrical elements of the stent are preferably plastically deformed when expanded (except when nickel-titanium (NiTi) alloys are used as the elements) so that the stent remains in the expanded condition. Therefore, when non-NiTi elements are used, the elements must be sufficiently rigid when expanded to prevent the collapse thereof in use. With super-elastic NiTi alloys, the expansion occurs when the stress of compression is removed which relief causes the phase transformation of the material from the martensite phase back to the expanded austenite phase.
After the stent is expanded, some of the peaks and/or valleys may tip outwardly and become embedded in the vessel wall. Thus, after expansion, the stent does not have a smooth outer wall surface, but rather is characterized by projections which embed in the vessel wall and aid in retaining the stent in place in the vessel.
Other features and advantages of the present invention will become more apparent from the following detailed description of the invention, when taken in conjunction with the accompanying exemplary drawings.